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Japanese Journal of Applied Physics 57, 110304 (2018)https://doi.org/10.7567/JJAP.57.110304

Spin splitting in EuO(111)/Si(111) spin-ﬁlter tunnel junctions
with atomically sharp interface
Rento Ohsugi1*, Hiroo Omi1,2, Yoshiharu Krockenberger1, and Akira Fujiwara1
NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, JapanNTT Nano Photonics Center, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan1
2*E-mail: osugi.rento@lab.ntt.co.jp
Received August 8, 2018; accepted September 18, 2018; published online October 16, 2018
We demonstrate the tunneling in spin-split barriers made of ferromagnetic EuO grown on Si(111) substrates by molecular beam epitaxy. For 6 nm
thick EuO ﬁlms with high crystal quality and atomically sharp interfaces, we ﬁnd a barrier height lowering driven by the spin splitting below the Curie
temperature of 35 K. We determined the splitting energy to be 0.56 + 0.03 eV at 20 K which results in a spin polarization above 90%.
© 2018 The Japan Society of Applied Physics

T

he development of next-generation electronic devices1,2) demands for ways and means to generate
spin polarized states in Si, e.g., spin metal–oxide–
semiconductor ﬁeld-eﬀect-transistors (spin-MOSFETs3)). An
eﬀective way of generating spin polarized states is the use of
the spin ﬁltering eﬀect in ferromagnetic insulators (FMI).
These materials have spin-split barrier heights (ϕ↑ and ϕ↓)4)
where down-spin states tunnel through a barrier higher thanthat of up-spin states. This leads to a considerable diﬀerenceof up- and down-spin transmittances due to the exponential
dependence of the tunneling probability on the barrier heights.
As a result, the tunneling barrier with a large spin-splittingenergy eﬀectively polarizes up- and down-spin states.
EuO is known to show large (97%) spin polarizations (P)in FMI=metal junctions,5–7) and more importantly, it can begrown epitaxially on Si without impeding the Si surface. EuO
crystallizes in a simple rock-salt cubic crystal structure[Fm3m;
space group (S.G.), No. 225, a = 5.144 Å8)] and
therefore a well matched material with Si (Fd3m;
S.G.,
No. 227, a = 5.431 Å). Thus, the EuO=Si junction has great
potential as an ideal spin ﬁlter for Si spintronics compared topreviously reported Fe=MgO=Si junctions giving P = 53%.9)
To realize high P, the formation of the EuO=Si structure witha high crystalline quality and a sharp interface is crucial forthe undistorted transport of spins through the structure.10,11)
One of the diﬃculties in forming EuO=Si junctions is
known to be related to the Eu–O2–Si thermodynamics.12,13)
The precise control of the oxygen pressure during the growthof EuO on Si is inevitable14) to avoid the formation of antiferromagnetic Eu3O415) and paramagnetic Eu2O3.16) Moreover, Eu-silicides17) and Eu-silicates18) are likely formed at
EuO=Si interfaces. It has been found that such intermixing
eﬀects can be suppressed by the growth of EuO on adatomterminated Si surfaces.14,17) Recently, epitaxial growth of
EuO has been reported with Eu-terminated Si(100)-(5×1)17)
and Eu-terminated Si(111)-(2×1)19) to which a surface
structure model has been proposed.20) However, there has
been no experimental demonstration of spin ﬁltering of EuO=
Si junctions, suggesting that the quality of the EuO thin ﬁlms
and interfaces are still insuﬃcient.
In this letter, we present a systematic study of the epitaxial
growth of EuO on Si with a variety of Eu-terminations
on Si(111). Here, special care has been taken to assure
atomically sharp interfaces between EuO and Si. To evaluate
the spin ﬁltering of EuO=Si junctions, we fabricated tunnel
junction devices. These devices exhibit a clear temperaturedependence of the tunnel current through ferromagnetic EuO
epitaxially grown on Si.
These EuO thin ﬁlms were grown by reactive molecularbeam epitaxy (MBE 21)). As an oxidizing agent, we usedmolecular O2 supplied by a nozzle located 15 cm below the Si
substrate. We used Eu metal and evaporated from a Knudsen
cell at cell temperature of 500 °C [Eu ﬂux 5.3 × 1014
atoms=(cm2·min)]. Si(111) substrates were prepared by a
wet chemical process22) prior to installing them in the MBE
chamber. For Si surfaces, it is known that a 7×7-Si reconstructed surface ensures wide ranging atomical ﬂatness.23)
Therefore, prior to the growth, we ensured the existence ofsuch a 7×7-Si surface by reﬂection high-energy electron
diﬀraction (RHEED) while heating the substrate to a temperature of TS = 760 °C under vacuum (1.8 × 10−10 Torr). We
deposited a thin Eu-termination layer on the Si(111) surface
at TS = 720 °C. Then, we decreased TS to 510 °C, and subsequently supplied O2 at a pressure of 8.0 × 10−9 Torr, corresponding to a ﬂux of 4.8 × 1013 O2 molecules=(cm2·min). Eu
atoms and O2 molecules were simultaneously supplied during
the growth of the EuO layers. After the growth of EuO ﬁlms,
the TS was rapidly lowered to room temperature.
The cubic crystal structure of the EuO ﬁlms was identiﬁedby X-ray diﬀraction (XRD). Figure 1(a) shows a typicalexample of the XRD proﬁle from the ﬁlm grown on a
2×1 Eu-terminated Si(111). The observed diﬀraction peaks
coincide with EuO(111), (222), and (333) peaks, respectively,8) and there are no indications of other impurity phases.
These peaks prove that the EuO ﬁlms are (111)-oriented,
parallel to the (111) axis of Si. By analyzing the position ofthese peaks, a cubic lattice constant of the EuO thin ﬁlms wascalculated by the Nelson–Riley method24) to be a = 5.24 ±0.05 Å. This lattice constant is slightly larger than that ofbulk EuO (a = 5.144 Å8)), suggesting that the EuO ﬁlms are
strained along the 〈111〉 direction with a tensile strain of
1.89%.
The growth of EuO ﬁlms can be inﬂuenced by the degreeof Eu-termination on Si. We evaluated the EuO(222) diffraction intensities for diﬀerent Eu-terminations [Fig. 1(b)]. It
was found that the 2×1 Eu-termination with a thickness of
1.87 ± 0.10 Å resulted in the highest XRD peak intensity.
The other terminations, e.g., 7×1 and 3×1 resulted in signiﬁcantly lower XRD intensity. This is also true if the Eutermination is absent. These results clearly indicate that the
crystalline quality of EuO strongly depends on the surface
termination schemes and it is the best with the 2×1 Eu-

110304-1© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 110304 (2018)(a)R. Ohsugi et al.

(b)

(a)

(d)

(c)

(b)

EuO (111)
EuO
Si

Si (111)(e)

EuO 5.0 nm

Fig. 1. (Color online) (a) The XRD proﬁle of the 28.2 nm thick EuO ﬁlm
grown on a Si(111) substrate with a Eu-termination of 1.87 Å. Diﬀraction
peaks of the Si substrate are marked with asterisks. (b) EuO(222) diﬀraction
peak intensities as a function of the eﬀective thicknesses of the Eutermination layers. The Eu layer thickness was determined by the quartz
crystal micro balance. The EuO thicknesses were constant at 28.2 nm.
Data points are labelled by the surface reconstruction matrix prior to the EuO
growth. The surface reconstructions are determined by RHEED measurements (not shown). (c) is cross sectional STEM image of a EuO=Si(111)structure. The accelerating voltage was 200 kV. All cross-sections are along{110} planes. (d) is enlarged image of the boxed region in (c). The black,
red, and blue circles are showing the positions of Eu, O, and Si atoms,
respectively. As the positions of the lightweight O2− ion are invisible, we
assumed them from the relative position of Eu2+ ions. (e) is the RHEED
pattern taken from the surface of a 5 nm thick EuO ﬁlm. The incidental

direction of the electron beam was ﬁxed to ½110.

termination, which is a surface structure that allows forcomplete coverage of the Si surface.20) Therefore, the formation of a 2×1 Eu-termination layer suppresses Eu–O–Si
ternary reactions. On the other hand, a thicker Eu-termination(>2 Å) results in the formation of islands of Eu-silicides,17)thus degrading the interface crystalline quality.
Next, we characterized the EuO=Si interface by cross-sectional scanning transmission electron microscopy (STEM).
The STEM image in Fig. 1(c) shows that the epitaxial ﬁlm iscomposed of single phase EuO. First of all, the interface area
between EuO and Si is abrupt. Moreover, the STEM image
shows a contrast variation in the vicinity of the interface. We
therefore analyzed the interface region by energy dispersive
X-ray analysis and found an oxygen concentration higher
than what is expected for stoichiometric EuO. The interface
area is further magniﬁed in Fig. 1(d) to highlight the atomic
conﬁgurations of the interface formed by EuO growth on the
2×1 Eu-terminated Si surface. One can recognize that the Eu
ions are located on the silicon atoms with a lateral Eu–Eu
distance ∼3.21 Å, indicating that the EuO(111) ∥ Si(111) and

Fig. 2. (Color online) (a) Temperature dependence of J–V curves takenfrom the EuO=Si junction with EuO thickness of 6 nm. The dashed lines
shows ﬁtting curves by Simmons model. A microscope image from top viewof the junction is exhibited in the inset (i). The inset (ii) schematically shows
the layer structure and the measurement conﬁguration of the device.
(b) Temperature dependence of the junction resistances taken at appliedbias voltages of −0.6 and −1.0 V in the J–V curves.

EuO〈110〉 ∥ Si〈110〉 alignments originate from the interface. Moreover, no intermixing layer is discernible, emphasizing the atomically sharp interface between EuO and Si.
Furthermore, as shown in Fig. 1(e), to ensure a uniform
growth of EuO, we monitored the growth in real-time by
RHEED. In Fig. 1(e), we show a RHEED image taken after
growth of a 5 nm thick EuO ﬁlm. The streak pattern indicatesatomically ﬂat surface of the EuO ﬁlm has formed, which isin contrast to the spotty RHEED pattern in the previous
study.19) This strongly suggests that EuO was grownuniformly in our sample, which will be important for the
fabrication of tunnel junction devices with thin ferromagnetic
EuO barriers.
The spin polarized tunneling current (J↑ or J↓) throughsuch device contains the information related to the barrierheights as well as the degree of P. We fabricated metal=EuO=
Si tunnel junctions with a diﬀerent areas S (S = 0.64, 1.00,
2.25 × 10−2 mm2) by using patterned Au=Sc as contact metal
on EuO and Si. The Si substrate used here is n-type (P doped)with a doping density of ∼1016=cm2. The layer structure[inset (ii) in Fig. 2(a)] is Au (120 nm)=Sc (30 nm)=EuO (6nm)=Si (380 µm)=Sc (15 nm)=Au (15 nm). Note that we selected Sc as a contact material because Sc ensures Ohmic
contact with Si and good adhesion with EuO. The devices
were mounted on a cooling stage in a probe station to measure the temperature dependence of the current density–
voltage (J–V ) curves. J–V measurements were performed by110304-2© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 110304 (2018)

R. Ohsugi et al.

a two terminal conﬁguration with a source measure unit
(Keithley 2635B). As the series resistances (∼500 Ω) of the
Sc=Si interface and the Si substrate were smaller by eightorders of magnitude compared to the EuO tunnel tunnel
resistance. It is safe to assume that the voltage drop measured
occurs only at the EuO tunnel barrier. We varied the sample
temperature from 17 to 80 K while simultaneously acquiring
J–V data. We determined the spin splitting of the barrierheights and TC of EuO barriers by investigating the temperature dependence of tunnel resistance at zero magnetic ﬁeld.
This method is commonly used to determine spin splitting
barrier heights of FMI=metal junctions as shown in previous
studies,6,7) where spin splitting was detected as abrupt resistance change below TC and the splitting energy could be
extracted by means of ﬁttings of J–V curves with proper
tunnel current models. Here, we focused on the J–V measurements of 6 nm thick EuO ﬁlms as EuO ﬁlms with athickness of 3 nm did not show the spin splitting whereas theresistance for larger thicknesses exceeded our measurement
range.
Figure 2(a) shows the J–V curves of the device with S =1.00 × 10−2 mm2 at three diﬀerent temperatures, i.e., 17, 29,
and 35 K. The current densities exhibit a nonlinear increase
with bias voltage. This indicates that the EuO (6 nm) acts as atunnel barrier and the tunnel probability is bias dependent.
Such a bias dependency can be analyzed by Simmonsmodel.25) What is striking in Fig. 2(a) is that the tunnel
current signiﬁcantly increases as the temperature decreases,
which cannot be explained by normal tunnel barriers andstrongly suggests that the barrier height rapidly change as
the temperature varies. Note that, a similar temperaturedependence was reproduced for devices independent of S. As
the J–V response is independent of S, it supports that theobserved behavior is intrinsic nature of EuO barrier.
Using the J–V data, we determine the tunnel resistances andplotted it as a function of temperature for two diﬀerent bias
voltages in Fig. 2(b). Independent of the bias voltage, theresistance monotonically increases above 35 K and suddenly
drops by 1=5 below 35 K. The abrupt reduction of the resistances is associated to the reduced tunnel barrier heights offerromagnetic EuO below TC.6,7) Below TC, the conductionband bottom of the barrier splits into two spin-dependentlevels (up- and down-spin)4) with a splitting energy 2ΔE
as shown in Fig. 3(b) later. The spin-split barrier heights
relating to the ΔE are deﬁned as ϕ↑ = ϕ(TC) − ΔE and ϕ↓ =ϕ(TC) + ΔE. Here, the 2ΔE changes proportionally to themagnetization M as ΔE(T ) ∝ M(T)=M(T = 0).6) Therefore,
the ΔE(T ) becomes smaller with increasing temperatures,and it is zero above TC, and therefore, ϕ↑(T ) = ϕ↓(T ) in theparamagnetic state of EuO. The TC of the ferromagnetic layer
embedded in the spintronic device [inset (i) in Fig. 2(a)] was
TC = 35 K, which is lower than that of bulk EuO (69 K)26) andthe 40 nm thick EuO ﬁlm (68 K) of the previous study.19) As it
is previously reported that oxygen-rich EuO ﬁlms drasticallyreduce TC to be less than 15 K,27,28) consequently, the oxygenrich layer [see Fig. 1(c)] occupying a volume of 20% in ourthin EuO ﬁlms, can cause a reduction of TC.
Next, we analyzed the spin-dependent barrier heights
to estimate eﬃciency of spin ﬁltering driven by the spin
splitting 2ΔE. This requires an estimation of the barrierheights which we determine from J–V curves using Simmons

(a)

(b)

(c)

Fig. 3. (Color online) (a) Temperature dependence of tunneling barrierheights determined by the ﬁttings of J–V curves in Fig. 2(a). The red andblue lines show the barrier heights for up- and down-spins, respectively.
Above TC, the tunneling barrier heights are spin-independent due to theparamagnetic state of EuO. Inset: the temperature dependence of the eﬀective
tunneling mass derived by Simmons model. (b) A sketch of the barrier height
lowering due to ferromagnetic transition of EuO. (c) The P values of thetunneling current density under reverse bias voltages of −0.6 V (circle) and−1.0 V (square) below TC.

tunnel model.25,29) In Simmons model, the current density J isgiven asJ ¼ J" þ J#


 

1 X
eV
eV
¼
A  ðEÞ 
exp B  ðEÞ 
2 ¼";#
2
2


 

eV
eV
  ðEÞ þ
exp B  ðEÞ þ
;
2
2
 e
4d pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2m m0 ;
A¼
; B¼
2
2hd
h
where J↑ and J↓ are the spin-dependent current densities, e isthe electron charge, m0 and m+ are the mass of free electron
and the eﬀective tunneling mass, respectively, h is Planck’s
constant, d is the thickness of the EuO barrier, and V is theapplied voltage. Above TC, for the numerical ﬁtting process,we set ϕ↑(T ) = ϕ↓(T ) and m+ as ﬁtting parameters. This
results in a barrier height of 2.33 eV and m+ = 0.14 at TC.
Then, below TC, we ﬁxed the barrier height ϕ(TC) = 2.33 eV,
whereas ΔE and m+ are ﬁtting parameters. Moreover, we kept
the tunneling parameters A, B constant using a ferromagnetic
thickness of d = 6 nm. The ﬁtting curves (dashed line)
together with the measured data are shown in Fig. 2(a) in
the reverse bias region where the spins are injected from EuO
into Si, supposing that the device operates as a spin ﬁlter of
Si. The m+s determined by this method are m+ = 0.14 ± 0.02,
which is smaller than that of bulk Gd:EuO (m+ = 0.4),30) but

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Jpn. J. Appl. Phys. 57, 110304 (2018)

R. Ohsugi et al.

comparable to other transition metal oxides e.g., HfO2 (0.15)
and Y2O3 (0.25).31)
Figure 3(a) shows the temperature dependence of theobtained barrier heights. The barrier heights above TC are
2.08–2.57 eV. Using the work function of Sc at 3.5 eV, thebarrier heights are converted into EuO’s electron aﬃnities of
0.93–1.42 eV. Reported values for the EuO electron aﬃnity
range between 0.6 to 1.8 eV32,33) and our values are in thisrange. More importantly, it is clearly shown that ΔE grows
rapidly below TC, consistent with the rapid decrease of thetunnel resistance. The splitting energy 2ΔE reaches 0.56 ±0.03 eV at 20 K, which well coincides with the previously
reported value of 0.6 eV obtained from Auger spectroscopy at
20 K.34) The large spin splittings in tunneling barrier heights
observed here suggest that P might be close to 100%. To
evaluate P of our EuO=Si junction, we calculate P = (J↑ −J↓)=(J↑ + J↓) and found it to be larger 90%. Inﬂuences of spin
ﬂip processes due to possible imperfections at the EuO interface are neglected for simplicity. The temperature dependence of P is plotted in Fig. 3(c). Below 32 K, P is larger than
90% [Fig. 3(c)]. At 17 K, the maximum P of 98 ± 0.5% andthis is comparable to the reported P of 97% in the Al=EuO=Y
junction.6)
In summary, we surveyed the inﬂuences of various Euterminations on the Si(111) surfaces to improve on the crystalline quality of epitaxially grown EuO thin ﬁlms. We foundthat the 2×1 Eu-termination is optimal to form atomically
sharp interfaces between Si(111) and EuO(111). Using thehigh crystalline quality EuO(111) tunnel barrier with theatomically sharp interface, we investigated the temperaturedependence of the tunnelling barrier heights of EuO(111) on
Si. We observed that, below TC, the tunneling barrier heightis reduced and this corresponds with the spin ﬁltering
phenomena. The evaluated spin splitting in the EuO ﬁlm is
0.56 eV and the estimated spin polarizations are larger than90%. More work should be done to investigate the electronaﬃnity and the J–V characteristics under magnetic ﬁelds.
Acknowledgment Authors thank Dr. Mitate of NTT Advanced Technology Corporation for observing cross-sectional STEM images.

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